1. Field of the Invention
The present invention relates to a zoom optical system and an imaging apparatus using the zoom optical system.
2. Description of the Related Art
In recent years, there has been an increased demand for small high resolution zoom optical systems, such as digital cameras, and cellular phones with cameras.
With a small high-resolution zoom optical system, zooming is commonly performed by moving a plurality of lens groups (lens units) in the optical-axis direction as to a photo-accepting surface (CCD). At this time, a zoom method for moving lens groups (lens units) in the object direction causes the entire optical length (length from a first lens surface to the image plane) to be lengthened, and this contributes to restricting reductions in size of the entire lens system.
In order to reduce the above factor, heretofore, an optical system employing optical elements called Alvarez lenses has been discussed for changing the power of the entire system by moving the optical elements in a direction different from the optical-axis direction (see U.S. Pat. Nos. 3,305,294, 3,583,790, and Optics Vol. 29, No. 3 (2000)).
Subsequently, various types of zoom optical systems for performing zooming using these Alvarez lenses have been discussed (see Japanese Patent Laid-Open No. 1990-119103).
With the optical system discussed in U.S. Pat. No. 3,305,294, power is changed by providing two lenses, which can have a surface represented with a tertiary function, and shifting these lenses in the direction different from the optical-axis direction. This optical system does not send out the lens groups (lens units) in the optical-axis direction, so employing this for a zoom optical system can reduce the entire lens length.
Also, the optical system discussed in U.S. Pat. No. 3,583,790 reduces an aberration by providing lenses, which can have a curved surface represented with an order term higher than a tertiary term, particularly a quintic term.
Further, in Japanese Patent Laid-Open No. 1990-119103 there is discussed an example in which two lens are employed for a zoom optical system. Subsequently, a theory for changing power while maintaining an image point in a steady manner has been discussed by disposing at least the two above lenses.
On the other hand, Optics Vol. 29, No. 3 (2000) describes an optical system including a rotational asymmetric optical element. This optical system has no common axis (optical axis), which is different from a normal coaxial lens system. Such a non-coaxial optical system is called as an Off-Axial optical system, which can be defined as an optical system including a curved surface (Off-Axial curved surface) where when assuming that the route in which the ray passing through the image center and the pupil center traces is taken as a reference axis, a surface normal at an intersection with the reference axis of constituent surfaces is not present on the reference axis. In this case, the reference axis has a folded and bent shape. Accordingly, this needs to employ a paraxial theory based on the Off-Axial theory other than the paraxial theory of a coaxial system such as usually employed for calculation of the paraxial amount. The optical principle of the method thereof has been introduced in the optics Vol. 29, No. 3 (2000), which is performed by calculating 4×4 determinants based on a surface curvature, for example.
With U.S. Pat. Nos. 3,305,294, 3,305,294, and 3,583,790, descriptions have been made regarding a method for changing power with a pair of rotational asymmetric lenses and correction of an aberration, but they cannot maintain an image plane in a steady manner when changing power.
Also, with Japanese Patent Laid-Open No. 1990-119103, the principle for changing power while maintaining an image point in a steady manner has been described, but has not reached the design level of a zoom optical system for obtaining an actual appropriate image by performing correction of an aberration.
In order to configure a zoom optical system with Alvarez lenses, it can be necessary in some circumstances to configure the system so as to have a steady image plane even at the time of zooming, and reduce the aberration fluctuation due to zooming.